It is of course essential that the utmost care be taken to remove by rinsing or distillation all traces of ammonia from apparatus employed. Water which has been distilled till free from ammonia should alone be used in estimations and preparations of solutions, and the alkaline permanganate should lie boiled for a short time when made to expel ammonia.
“Oxygen” Process. This is a useful process when comparing waters of similar origin. It is probably a more reliable measure of the putrescent matter present than the total organic contamination. It is essential that the oxidizing agent potassium permanganate be added in excess and allowed to stand three hours. The following method is very delicate (vide Dr Tidy on Potable Waters, ‘Chem. Soc. Journ.,’ January, 1879.)
Cleanse with sulphuric acid and with tap water two flasks and place in one 500 septems (1⁄20 gall.) of the water, in the other an equal quantity of distilled water. Add to each 20 septems (140 gr.) of sulphuric acid (1 part pure acid to 3 of distilled water) and 20 septems of potassium permanganate and allow to rest for 3 hours. Then add to each flask a couple of drops of an aqueous solution of potassic iodide (1 in 10) when iodine is liberated equivalent to the amount of permanganate unacted on by the waters. Observe the amount of a sodic hyposulphite solution (5·4 gr. in 7000 gr.) which must be added to each to remove this free iodine (judging of the exact spot by adding towards the end of the experiment a few drops of starch).
The strength of the potassic permanganate solution is 2 gr. of the salt in 7000 gr. to 1⁄10 gall.; therefore the 20 septems will contain ·04 gr. permanganate, equivalent to ·01 of available oxygen. The experiment (A) with the amount of hyposulphite used up for the blank distilled water shows the amount of hyposulphite equivalent to 20 septems or ·01 gr. of oxygen. Therefore the amount of oxygen unconsumed in the water (B) to be examined was (B/A) × ·01 and the amount (C) actually used up was (A - B)/A × ·01 for 500 septems (1⁄20th gall). Then the oxygen consumed
per gallon would be A-B x ·2 / A. It is necessary to perform this standardising of hyposulphite with every series of experiment on account of its tendency to change. Dr Tidy recommends that in addition to the three hours’ experiment one of a single hour duration be executed. The higher the proportion of oxygen consumed in one hour to the oxygen consumed in three hours the worse the water.
Nitrites, sulphuretted hydrogen, and ferrous salts interfere with this test, and there appears to be a different ratio between the oxygen consumed and the amount of organic matter according to the amount of oxidation that has already taken place. The organic matter of deep wells is proportionately least acted upon.
Combustion methods.—The “Frankland and Armstrong process” consists in burning with oxide of copper in vacuo the residue left on evaporating the water, and collecting and measuring in a suitable gas apparatus the carbonic acid, and nitrogen, and nitric oxide proceeding from the organic matter. From these estimations are calculated the organic carbon and nitrogen.
This method, though forming the most accurate means of measuring organic contamination, is not in general use in consequence of the difficulties attending Dr Frankland’s method of analysis. Professor Dittmar and Drs Dupré and Hake have lately introduced processes by which the same results may be obtained without necessitating the use of expensive gas apparatus.
Dittmar’s Carbon.—Concentrate a suitable quantity (say 10,000 gr.) in a pear-shaped flask, and, after adding some saturated solution of sulphurous acid to expel carbonates and nitrates, evaporate to dryness in a glass dish on a water bath. Transfer the residue from the dish to a porcelain or platinum boat, and introduce it into the tail end of a combustion tube, filled three fourths of its length with oxide of copper, and having a roll of silver gauze in the front part of the tube. Previous to the boat being put in, this tube is heated to redness, and a stream of air, freed from carbonic acid, passed through it till the gas which comes out no longer renders clear baryta water turbid. The combustion tube has attached in front a small V-Shaped tube charged with chromic acid, dissolved in 60 per cent. sulphuric acid. To it is permanently fixed a small tube filled with calcic chloride, and in front of all is a small-weighed U-tube the first three fourths of which is filled with soda lime, and the other fourth with calcic chloride. On turning the gas on gradually from the front to the tail the residue is at last reached, and burnt in the stream of pure air. The carbonic acid given off, after being freed from sulphurous anhydride by passing through the chromic acid solution and of moisture by the
calcic chloride, passes into the soda lime tube and is absorbed. The increase in weight multiplied by 3⁄11 gives the amount of carbon in the amount of water taken.